This invention relates generally to a small printing device using a type wheel selection system and more particularly, to a small printing device in which typing wheels are set at a desired position by a selection device, a paper is printed and thereafter is passed to a take-up device. The typing wheels include a plurality of numerals, letters or characters symbols, or the like on the external peripheral surface. The printing device is used for point-of-sale and electronic cash register applications.
Predominately, conventional printing devices having typing wheels are constructed in such a way as to frictionally drive the selected typing wheels and to effect aligned printing. Thus, conventional printing devices are unfit for ink roller type applications making it necessary to use an ink ribbon. This is a significant disadvantage, for example, in an electronic cash register which is subjected to heavy duty usage. Further, U.S. Pat. No. 2,770,188 discloses a construction wherein a ratchet and a ratchet wheel are engaged with each other and driven in a type wheel system. In this construction, however, there is the drawback that the electromagnetic actuation device must be energized until completion of the printing in order to maintain engagement between the ratchet and the ratchet wheel. Also, there is another significant drawback in that a ratchet wheel is required for each typing wheel. These factors increase the cost of the device and make it complex to manufacture.
Furthermore, as disclosed, for example, in the U.S. Pat. No. 3,920,113, assigned to the present assignees, another conventional printing device is of a pressure type. The device includes a printing roller rotatably held by a bearing plate which is provided with a bearing portion of a circular slot configuration and which is fixed to a crank shaft which continuously rotates in one direction. This conventional system has a major advantage in that the printing roller is in contact with and rolls on the surface of a typing wheel having letters or characters thereon, so as to apply a pressure, thereby producing a high printing pressure with a low torque. Such a printer is capable of copying using carbonless duplicating paper. However, with such a construction, there is an inconvenience that the diameter of the locus of the outer periphery of the printing roller which rotates, must be increased as the surface of the typing wheel having letters or characters increases. Thus, the printing device having a characteristic feature that the letters or characters are of large size, has a major disadvantage in that the structure as a whole is large and much space is required.
This system has another major disadvantage in that the printing roller is positioned a long distance away from the surface of the typing wheel having letters or characters and therefore a long distance from the printing paper, other than at the time of actual printing, thereby making it difficult to insert the printing paper to a desired preselected position. In addition, in this construction it is necessary to move the printing paper over a long distance immediately after completion of printing in order to confirm the quality of the printed letters or characters. Thus, there is also another drawback in that the distance between the printed lines must be increased. In addition, this system has another disadvantage in that the torque when pressure is applied and printing is effected, must be in proportion to the number of letters or characters to be printed.
As a swinging system for pressure printing, there has conventionally been known a method whereby the same letter or character is pressed and printed twice within one reciprocating motion of a pressing member, or a following line is printed during a return motion. In this case, however, there is not only a serious disadvantage in that a shear in printing is caused when the same letter or character is pressed and printed twice within one reciprocating motion, but also another disadvantage in that the time required for the printing is twice as long as that usually required. Thereby, printing speed is decreased. When a new line is started within one reciprocating motion, the direction of movement relative to the printing paper reverses, and thus the space between the lines becomes irregular. This is also a serious disadvantage.
Thus, for the reasons enumerated above, there are various types of serious disadvantages which are difficult to overcome in the conventional pressure printing constructions.
What is needed is a small printer capable of printing letters or characters of large size, with regularly spaced and aligned lines, by pressure techniques.